This invention relates to an improved active load for a buffer amplifier.
In a basic prior art amplifier design, for example as illustrated in FIG. 1, a pair of emitter-coupled transistors provide non-inverting and inverting inputs. The collectors of the emitter-coupled transistors are each coupled to a current source and one of the collectors is coupled to the base of an output transistor which is coupled to the output load. The current source and the output transistor are considered the active load of the amplifier. In operational amplifier design, it is desirable to provide a high gain from input to output while maintaining a negligible offset voltage. The offset voltage is the voltage difference between the inverting and non-inverting inputs.
The current source for an amplifier is typically designed to provide equal currents to both collectors of the emitter-coupled transistor pair. However, the load current flowing through the output transistor into the load impedance causes a base current to flow in the output transistor. This base current is drawn from the current to the collector of one of the emitter-coupled transistors. This causes an unbalance in the currents to the two emitter-coupled transistors and thereby creates an offset voltage which is a function of the load current.